The success which Sir W. Herschel obtained in the construction of reflecting telescopes was unrivalled during many years ; but at length has been surpassed by Earl Reese, who has erected in the grounds of Birr Castle, Parsonstown, Ireland, two telescopes, in the lesser of which the speculum is of three feet aperture, and in the greater telescope 6 feet, or a reflecting surface of 28'274 square feet, being greater than that of Herschel's large telescope in the ratio of 7 to 3. Its focal length is 53 feet. It can be used either on the Newtonian or the Herschelian principle. The great tube is of wood hooped with iron, and is 7 feet in diameter and 52 feet in length. It is suspended between two lofty meridional walls of solid masonry, between which its upper end is allowed a considerable amount of lateral motion, so as to admit of taking up the view of a celestial object some time before its arrival on the meridian, and following it for some time after, with out displacing the lower end. This lower end is supported on a massive universal joint of cast iron, resting on a atone pier buried in the ground, and so counterpoised as to be easily moved in declination. The preparation of the apeculum, which weighs 4 tons, is described under SPECULUM. The stairs and galleries for the observers are supported by the western pier. The first gallery commands a view of objects at an altitude of 42°. It consists of a strong light prismatic framing sliding between two fixed ladders ; it is counterpoised, and can be raised to the required position by means of a windlass. There are three other galleries at the summit of the western pier, which com mand the heavens to five degrees below the pole. Each gallery is supported by beams which run between grooved wheels. For some of the achievements of this splendid instrument we may refer to a Memoir by Lord Rome, Phil. Trans.' 1850. See also NEBUL4E.
Mr. Lamella' reflector has a clear diameter of 2 feet, and 20 feet focal length ; it was originally erected at his residence near Liverpool. and in 1852 was removed to Malta, for the advantage of a clearer atmosphere and a lower latitude. By means of this instrument, Mr. Lassells
discovered two of the satellites of Uranus, one of Saturn, and one of Neptune. This instrument is erected under a revolving cupola 30 feet in diameter, carrying a stage for the observer. The image may be deflected towards the eye-glass, either by a small two-inch speculum, or by a prism. To prevent dew forming on the latter, a small piece of heated lead is placed in a case near it. Metal does not contract dew, but glass is peculiarly liable to deposit it; hence achromatics are generally furnished with a dew-tube, which is merely a brass cylinder blackened within.
In Mr. Nasmyth's reflector, the rays from the great speculum are received on a small speculum or prism, placed in the axis of the tube, between the focus and the great speculum, by which they are reflected at right angles, and the image is formed in a tube inserted in one of the trunnions, on which the instrument turns. The image is then viewed in the usual way by an eye-piece ; the advantage is, that while the great tube is moved in altitude, the side-tube is fixed, and the observer can survey the whole meridian or any other vertical circle without changing his position. The instrument is moved in azimuth by means of a turn-table, on which is the frame of the instrument and the seat for the observer, who can thus command every required motion, both in altitude and azimuth. The great tube has a length of 28 feet, and a diameter of 54 inches.
Telescopes for general astronomical purposes are now almost always mounted equatorially, and great improvements have been made in the mounting, the method of the Astronomer Royal being particularly recommended for its lightness, combined with extreme stiffness to resist both flexure and twist.
The history of the telescope may be assisted by reference to the following heads : EQUATORIAL; TRANSIT; SPECULUM; ACHROMATIC; LIOTIT ; LENS; APLANATIO LENS; ABERRATION; MICROMETER; EYE PIECE; ASTROLABE; QUADRANT; THEODOLITE; HELIOSTAT; HELIO METER; OPTOMETER; REFRACTION; DISPERSION.